LASER ADAPTABLE TO LIGHTWEIGHT CONSTRUCTION

Abstract: A laser assembly includes a lasing medium (12), associated reflector components (14, 16) defining a resonance cavity, and means (16) for outputting laser pulses therefrom. Means (18) is provided to pump the lasing medium with pulses of predetermined energy. There is a pumping circuit (20) for activating the pump means, and control means (22, 34, 35) associated with said pumping circuit for determining the energy of the pumping pulses.

Description: LASER ADAPTABLE TO LIGHTWEIGHT CONSTRUCTION Field of the Invention This invention relates to lasers and has particular though not exclusive application to the construction and actuation of eye-safe laser devices. Background Art In a known eye-safe laser assembly, an erbium glass rod of 75 mm length is pumped by a parallel flash lamp. The flash lamp power circuit includes one or more capacitive storage devices and a wound inductance in series with the lamp. The total weight of these assemblies, apart from any battery supply, is around 4 pounds (1.8 kg), a major contribution being from the inductance. The capacitors are typically also large in volume, and hence the flashlamp-pumped erbium glass configuration, although eye-safe and simple to operate, has limitations in application where small size and/or low weight are called for. Another problem encountered with erbium glass lasers is their susceptibility to thermal droop and the associated difficulty of achieving adequate control of then- output energy within various limits. For a given energy input, E jj erbium glass lasers exhibit a decaying energy output as the temperature of the rod increases with successive pulses, and the temperature impulse response of the rod for each pulse has a half-life of the order of 100 seconds. On die other hand, there is a m-iriin-uim output threshold for adequate operation, typically around 4mJ per pulse. There are also two upper thresholds - the legal limit for eye-safe operation (typically 8mJ per pulse) and a higher limit above which the laser is rendered non-functional by double pulsing. The problem was to achieve an adequate repetition rate at a desired output energy while applying input energy of the appropriate level without exceeding die legal limit and without incurring double pulsing. A further problem associated with prior art flashlamp-pumped solid state lasers has been their fragility. Employment of such lasers in high vibration or high impact load applications is not possible because either the glass flashlamp or the laser material will break. One approach to meeting some of the problems associated with flashlamp- pumped erbium glass eye-safe lasers has been to substitute diode pumping. Diode pumping has allowed elimination of the wound inductance, thus reducing die weight, has obviated the problem of double pulsing, and has proven more energy efficient than flash pumping. However, a much larger (in volume) capacitance is needed and a diode pump costs of the order of twenty times more than a flashlamp pump. Thus, while diode pumped erbium glass eye-safe lasers have been produced, these units have proven expensive and have remained relatively heavy and bulky. Prior art admits that small active volumes with short (e.g.30 mm) gain lengths can provide useful pulse outputs (e.g. 7 mJ in 30ns from erbium glass), but it completely ignores the strength benefits of carefully selected dimensions for the "fragile" components. This is because prior art assumes universally that thermal droop must be compensated for solely by increasing the gain length. Prior art therefore overcame thermal droop by making the laser longer. This renders it more fragile, and requires more energy, locking in passive LC pump networks which are bulky, heavy, electrically inefficient, and optically inefficient (because current density is not controlled). Summary of the Invention It is an object of the invention to provide a laser assembly which is adaptable to the economic production of an eye-safe laser assembly that is lighter, stronger, more robust and preferably also less bulky than eye-safe lasers hitherto available. It has been realised that a preferred embodiment of a lighter, stronger, more efficient, smaller, and less expensive eye-safe laser can be obtained by initially considering die following points: die mode volume of laser crystal required for useful operation (4 to 8 mJ), excluding thermal droop, is preferably less than 100 mm * - *1 the gain length (round trip) required for efficient operation is desirably 40 to 60 mm. - the stiffness (strength) of a laser crystal increases dramatically as its length is reduced for a given thickness. When the length/thickness ratio is less than 10, the crystal will withstand lOOg shock if correctiy supported. die stiffness of a glass flashlamp similarly increases as length is reduced for a given wall thickness. EπGlass has a stored energy lifetime which is typically ten times longer than conventional laser materials such as Nd.YAG. This allows EπGlass lasers to be pumped witii ten fold power reductions for die same stored energy and pulse output. The invention accordingly provides, in one aspect, a laser assembly, preferably an eye-safe laser assembly, comprising:. a lasing medium, associated reflector components defining a resonance cavity, and means for outputting laser pulses therefrom; means to pump the lasing medium with pulses of predetermined energy; a pumping circuit for activating the pump means, preferably including an electrolytic capacitor for storing successive charges releasable to generate said pumping pulses; and control means associated with said pumping circuit for determining d e energy of said pumping pulses, preferably in dependence upon a monitored temperature at a location in, on or adjacent die lasing medium and/or upon the pumped history of die medium witiiin a predetermined preceding time period. Preferably, die control means is further arranged for controlling the current level of said pumping pulses. The lasing medium is preferably an erbium glass rod, which is moreover preferably of length no greater than 40 mm, most preferably in the range of 10 to 30 mm. The pump means and lasing medium are preferably such that the maximum energy output is 8mJ per pulse. The electrolytic capacitor is advantageously less tiian half the weight of a conventional dielectric capacitor. The pumping circuit is, moreover, preferably a sold state circuit, most preferably without a wound inductive component or with only one or more miniature inductive components. The lasing medium is preferably small enough for the pumping discharge from the capacitor to entail a limiting current below 100A. The laser assembly preferably includes means to monitor the temperature at a location in, on or adjacent d e lasing medium. The temperature is preferably monitored at a location on die surface of the lasing medium. This temperature is related to die core temperature but a conclusion as to the core temperature is dependent on die temporal proximity of preceding pump pulses. The temperature impulse response of the medium exhibits a half-life of the order of 100 seconds. Hence, the control of the energy of each pump pulse requires knowledge of botii medium temperature and recent pump history. The control means preferably includes a pre-programmed table providing preferred pulse energy levels for given temperature and recent pumping history inputs, having regard to selected criteria such as minimum energy output for proper laser operation, maximum energy output for eye-safe operation, and maximum energy output if double pulsing is to be avoided. Where activation of die pump means requires a threshold priming current profile, eg where the pump means is a flash lamp, the control means and charging circuit may apply a continuous low-level simmer current to die pump means. Preferably, however, for portable batter-powered use of die assembly, the control means and pumping circuit may be configured to precede each main pump pulse with a simmer pulse which primes the pump means with the necessary threshold current profile and simmer current but at a current level well below the current level of the main pump pulse. Indeed, in a fourth aspect, the invention more generally provides a laser assembly, preferably an eye-safe laser assembly, comprising: a lasing medium, associated reflector components defining a resonance cavity, and means for outputting laser pulses therefrom; means to pump the lasing medium with pulses of predetermined energy, which pump means requires a threshold current profile for proper activation; and means to activate the pump means, which means is configured to precede each main pump pulse with a pre-simmer pulse which primes the pump means with die necessary threshold current profile and simmer current but at a current level well below the current level of the main pump pulse. Brief Description of the Drawings The invention will now be further described, by way of example only, with reference to the accompanying drawings, in -which: Figure 1 is a simple block diagram of a portable eye-safe laser assembly according to an embodiment of die invention; and Figures 2 to 5 are circuit diagrams for die pumping circuit indicated in Figure 1. Best Mode(s) of Carrying Out the Invention The illustrated laser assembly 10 includes a rod 12 of lasing medium, in this case erbium glass, reflector components 14, 16 defining a resonance cavity including rod 12, pumping means in the form of an elongate flash lamp 18, a pumping circuit 20 for the flash lamp, and a control circuit 22 including a microprocessor based CPU 29. The whole assembly is typically mounted within an outer housing 11, whose dimensions will be discussed subsequentiy. Erbium glass rod 12 is of a size, ie length, selected to allow the pumping energy requirement to be reduced to a level (eg 20 J) at -which capacitive components of the charging circuit may be electrolytic ratiier than dielectric. To tiiis end, die rod is typically 30 mm in length and botii it and flash lamp 18, which extends beside and for die full length of the rod, are retained within a cavity 25 provided by a solid aluminium casing 24 formed of two assembled halves 23a, 23b. The exterior of casing 24 exhibits a rectangular box profile but the interior comprises opposed polished longitudinally extending surfaces 25a, 25b which together define an ellipse in cross-section. The foci are coincident with the axes of rod 12 and lamp 18. Odier known cavity arrangements may of course be substituted, but it is thought that the solid box proposed may provide a sufficient combination of thermal capacity and tiiermal conductivity to prevent overheating of cavity 25. The reflector components are conventional and include a rotatable prism mirror 14 at one end, driven by motor 15, and a partially reflective mirror 16 at the other end, which also thereby provides a window for outputting generated laser pulses 8. Mirror 16 may be concave. These pulses traverse a further window 23 in housing 11. Pumping circuit 20 is configured for reliance on an external battery pack 27 and includes an electrolytic storage capacitor 26 but is otherwise provided by solid state components. In particular, the conventional wound inductance (the heaviest component of conventional flash pumped erbium glass lasers) is omitted. The solid state circuit may include miniature inductive components. The pumping circuit 20 draws a programmable current from the battery pack 27 to charge storage capacitor 26. The current drawn from the battery is programmable at sub-circuit 34 by the CPU 29 to optimise battery lifetime for portable applications. The voltage to which the capacitor is charged is also controlled by die CPU, giving accurate energy control. Combined with the vasdy reduced input energy requirements and die enhanced efficiency of the lamp pump, this feature multiplies battery life. When the storage capacitor is charged (0.1 to 10.0 sees, depending upon charge current selected), and die laser is fired, the capacitor energy is switched to die flashlamp via a current contiOl sub-circuit 35. Once an energy has been selected (i.e. capacitor voltage), the duration of the lamp discharge is controlled by current to the flashlamp which can be set and tiierefore controlled by die CPU. A few microseconds before die main current is switched to die flashlamp, a high voltage trigger (generated by die sub-circuit shown in Figure 5) is transmitted to break down d e lamp and establish lamp conduction with a low current level. This trigger technique is used to minimise noise feedback to die control electronics when the high current discharge begins. More detailed circuit diagrams for pumping circuit 20 are to be found in Figures 2 to 5. In the pumping circuit 20 die current control element is an IGBT 40 (Figure 3) controlled by a comparator whose inputs are the flashlamp current as sensed by a Hall effect device 42 or shunt resistance, and die required current at any particular time within the discharge, as output from the command circuit. The command circuit may be a simple look-up table of time dependent current or an actively computed current level. The pumping circuit 20 obtains power through a switch mode power supply (Figure 4) connected directiy to the battery. The battery current limit is determined from time to time by the central processor 29 so as to smooth the current demand on the battery from all sources. This information is fed to one input of a comparator which in turn controls the step-up transformer of the switch mode power supply. The second comparator input is drawn from die resistor which senses current drawn from die battery. The circuit 20 acts as a controlled current source for the lamp. A major advantage over passive LC circuits is that the vast majority of the stored energy is released, vs a minority for passive circuits. Sub-circuits 34, 35 may be of any known suitable form incorporating standard solid-state integrated components. Control circuit 22 is incorporated to provide intelligent contiOl of the energy level of the pump pulses in dependence upon monitored temperature and/or upon the pumped history of the medium within a predetermined preceding time period. To tiiis end, a temperature sensor 28 is mounted to d e surface of rod 12. Recent pumping history is provided in a simple manner by maintaining a continuously updated memory record 30 in the microprocessor of the number of pumping pulses fired by the flash lamp in the immediately preceding two minutes: this time is selected to be longer than the thermal time constant (eg 100 sec) of the temperature impulse response of the erbium glass rod. A more complex history (eg summation of instantaneous responses for recorded pulses within a preceding time period) could be maintained if desired for an enhanced level of history. The control circuit maintains a pre-programmed "look-up" table 32 of preferred flash pumping pulse energy levels in relation to monitored rod surface temperature at sensor 28 and to d e recent pump history as indicated by d e contents of record 30. Look-up table 32 is based on die empirically established energy-temperature thermal droop diagram for die specific erbium glass rod 12. For any given pump energy input E^, the output energy of successive pulses diminishes witii rising rod core temperature unless there is a sufficient delay (eg two minutes) which takes account of the temperature impulse profile for the rod. Thus, to maintain energy output at a practical repetition rate and/or above the minimum operating threshold E Q JJJJU, the energy input of successive pulses must be increased and/or die current density of the lamp discharge, and die discharge time, varied. However, in an eye- safe environment, this must be done without exceeding the eye-safe threshold E omax , and in general without incurring double pulsing, which occurs above anotiier, usually higher, threshold E oc ip. For example, for a typical erbium glass rod, E nm j n may be 4mJ, E omax 8mJ and E 0(j p lOmJ. Thus, table 32 is designed to select die most appropriate EQ given the monitored temperature and die recent pump history having regard to selected criteria such as minimum energy output for proper laser operation, maximum energy output for eye-safe operation, and maximum energy output if double pulsing is to be avoided, die recent pump history taking account of differences between the monitored rod surface temperature and die core temperature. The preferred EJN may of course be based on differing criteria and/or differing priorities, depending on d e application of die laser. Circuits 20, 22 are also configured so tiiat each main pump charging pulse applied to flash lamp 18 by sub-circuit 35 is preceded by a simmer pulse which primes the lamp with the necessary threshold current profile and simmer current, but at a current level well below the current level of the main pump pulse. An optimum laser assembly constructed in accordance with Figure 1 has been found to have a total weight, including an outer aluminium housing, of the order of one pound (0.45 kg). The housing 11 of the optimum assembly has dimensions of 110 mm x 80 mm x 30 mm. This compares with a typical commercial eye-safe erbium glass laser, which weighs around four pounds (1.8 kg) and is of ten times greater volume. It will be apparent that a laser assembly in accordance with die invention has wide apphcation where a laser is required as part of personally carried or otiierwise portable equipment, eg in laser rangefinders for infantry weaponry. The advantages of the preferred embodiment can be further appreciated by taking account of the following interrelated considerations: - the smaller the laser, the stronger its "fragile" parts become an optimised (minimal) size requires less than 13 Joules of energy input (E jj ) to produce useful output, before thermal droop such low E jj requirements can be met by ultra-light and small energy storage components such as electrolytic capacitors (capacitor 26 in Figure 1) flashlamp efficiency studies show tiiat for a properly designed flashlamp, efficiency of pumping will depend upon current density optimised current densities for optimised flashlamps pumping low energy (i.e. less than 13 Joules) will be less than 100A at any instant currents below 100A (peak) can be actively controlled (at sub-circuit 35) by state-of-the-art solid state components such as IGBTs - with die current controlled at sub-circuit 35, the flashlamp efficiency can be increased to optimum, reducing die onset of diermal droop, and minimising its effect with both the current and energy controlled at sub-circuits 34, 35, the entire pumping circuit 20 can be adjusted to compensate for thermal droop by using CPU 29 to increasing pump energy, or to alter the temporal characteristics of die current discharge, or both active current control removes the need for die largest and heaviest component of conventional laser power supplies, the wound inductor active current control allows almost all d e energy stored in the energy storage capacitor 26 to be utilised, whereas a conventional passive LC circuit releases a minority of the stored energy. This feature allows further component shrinkage. the system memory 30 which as mentioned preferably retains information for longer than the diermal time constant (e.g. 100 sec) of d e laser medium, is used to adjust pumping parameters to stabilise output against thermal droop.

Claims: CLAIMS 1. A laser assembly comprising: a lasing medium, associated reflector components defining a resonance cavity, and means for outputting laser pulses therefrom; means to pump the lasing medium with pulses of predetermined energy; a pumping circuit for activating the pump means; and control means associated with said pumping circuit for determining die energy of said pumping pulses. 2. A laser assembly according to claim 1, wherein the energy of said pumping pulses is determined in dependence upon a monitored temperature at a location in, on or adjacent die lasing medium and/or upon the pumped history of the medium within a predetermined preceding time period. 3. A laser assembly according to claim 2, wherein said control means includes a pre-programmed table providing preferred pulse energy levels for given temperature and recent pumping history inputs having regard to selected criteria. 4. A laser assembly according to claim 3, wherein the selected criteria include minimum energy output for proper laser operation, maximum energy output for eye- safe operation, and maximum energy output if double pulsing is to be avoided. 5. A laser assembly according any preceding claim, wherein said control means is further arranged for controlling the current level of said pumping pulses. 6. A laser assembly according to claim 1, wherein said pumping circuit includes an electrolytic capacitor for storing successive charges releasable to generate said pumping pulses. 7. A laser assembly according to claim 6, wherein the lasing medium is small enough for the pumping discharge from said capacitor to entail a limiting current below 100 A. 8. A laser assembly according to claim 1, wherein said pumping circuit is a solid state circuit. 9. A laser assembly according to any preceding claim, wherein said laser pulses output by the assembly are such that d e laser assembly is eyesafe. 10. A laser assembly according to any preceding claim, wherein the pump means and lasing medium, and said energy of the pumping pulses, are such that the maximum energy output of said laser pulses is 8mJ per pulse. 11. A laser assembly according to any preceding claim, wherein said lasing medium is an erbium glass rod. 12. A laser assembly according to any preceding claim, wherein said lasing medium is a rod of length no greater than 40 mm. 13. A laser assembly according to claim 12, wherein said length is in the range of 10 to 30 mm. 14. A laser assembly according to any preceding claim further including means to monitor the temperature at a location in, on or adjacent die lasing medium. 15. A laser assembly according to claim 14, wherein said temperature monitor means is disposed to monitor the temperature at a location on die surface of the lasing medium. 16. A laser assembly according to any preceding claim, wherein the control means and pumping circuit are configured to precede each main pump pulse with a simmer pulse which primes the pump means with the necessary threshold current profile and simmer current but at a current level well below the current level of the main pump pulse. 17. A laser assembly comprising: a lasing medium, associated reflector components defining a resonance cavity, and means for outputting laser pulses therefrom; means to pump the lasing medium witii pulses of predetermined energy, which pump means requires a threshold current profile for proper activation; and means to activate the pump means, which means is configured to precede each main pump pulse with a pre-simmer pulse which primes the pump means with the necessary threshold current profile and simmer current but at a current level well below the current level of the main pump pulse. 18. A laser assembly substantially as hereinbefore described witii reference to the accompanying drawings.

Inventors: GREENE BENNY ALLAN, GREENE, BENNY, ALLAN
Applicants: ELECTRO OPTIC SYST PTY LTD, GREENE BENNY ALLAN, ELECTRO OPTIC SYSTEMS PTY. LIMITED, GREENE, BENNY, ALLAN
Application year: 1994
Main classification: H01S3
IPC codes: H01S3